An experimental study of the effect of water-soluble fluorescent surfactant on the interfacial wave characteristics of annular flows*

We study experimentally the effects of surfactants on gas-liquid annular flows. We present a novel method to prepare water-soluble fluorescent surfactant solution and its detailed characterisation. In the presence and absence of this fluorescent surfactant, we conduct a detailed study on air-water annular pipe flows. The liquid and gas Reynolds numbers range between 500 to 1375 and 0 to 40000, respectively. We perform structured planar laser-induced fluorescence (S-PLIF) and capacitance probe measurements to accurately obtain film-thickness measurements and reveal the temporal characteristics of the waves. We further explore the differences on the gas entrainment rates, entrainment depths, and size of the bubbles in the liquid films for both cases. Diagnostic methods to track surfactant concentration are also currently being developed.